1. Field of the Invention
The present invention relates to power electronics and variable frequency switching regulators, and more particularly to open and closed loop steady state frequency control of variable frequency switching regulators.
2. Description of the Related Art
Variable frequency switching architectures, such as, for example, synthetic ripple regulators, hysteretic regulators, constant on/off switching regulators, etc., are popular solutions for buck-type switching regulators used in various electronic devices, such as notebook computers and the like. Such topologies have the ability to respond quickly to transient events (e.g., significant load changes) without having to wait for a clock pulse or the like. Generally, such regulators operate in continuous conduction mode (CCM) at moderate to heavy loads and discontinuous conduction mode (DCM) at light loads. System designers prefer fixed frequencies in CCM so that filter components may be optimized and electromagnetic interference (EMI) minimized. The steady state operating frequency of such variable frequency switching topologies, however, varies widely with operating conditions, such as the output voltage, the input voltage, load conditions, etc. Many variable frequency switching regulators exhibit poor CCM steady state frequency control despite attempts to control the steady state frequency with open loop circuits. It has been a challenge, therefore, to accurately control the steady-state frequency of variable frequency switching regulators.
Various factors tend to affect the operating frequency of conventional regulators, such as inaccurate input voltage or output voltage sensing, changes of the loop compensation, changes of the output filter, comparator and driver propagation delays, inaccurate time constant matching, load changes (particularly for constant on-time switching devices), etc. Input voltage sensing is especially poor on devices that sample and hold the voltage of the phase pin to indirectly sense the input voltage. In many configurations, therefore, an addition pin is provided on the controller for receiving the input voltage to avoid sensing inaccuracies. An error amplifier with high bandwidth causes excessive ripple on the compensation signal, which causes the switching frequency to change with changes of the loop compensation or output filter. The switching frequency of constant on-time switching regulators tends to change with changes of the load. Although various regulator controllers employ extensive open loop circuitry in an attempt to stabilize the steady state switching frequency, the switching frequency control has still been poor. Some ripple control topology schemes have been able to improve the CCM frequency but at the expense of reduced transient response.
It is desired to improve the frequency control of many different types of variable frequency switching regulators without sacrificing transient performance.